The present invention relates to an ink-jet printer which performs printing on paper sheets by jetting ink.
In recent years, serial ink-jet printers are widely spreading for personal use. A serial ink-jet printer includes a platen for holding a paper sheet and a print head for jetting ink onto the paper sheet on the platen. The print head prints an image for one row on the paper sheet by jetting ink while moving in a main scanning direction parallel to the axis of the platen. The platen feeds the paper sheet in a sub-scanning direction perpendicular to the main scanning direction each time the image for one row is printed.
The print head has a line of ink-jet nozzles each for jetting ink to form one dot. As the ink-jet nozzles are repeatedly used, they become clogged with dust or ink unnecessarily present in the nozzles. In particular, clogging occurs frequently in a multicolor print head which has a greater number of ink-jet nozzles than a monochrome print head. To prevent clogging, the ink jet nozzles are cleaned by periodical maintenance.
In the maintenance, the print head is moved outside a paper holding region of the platen in the main scanning direction. At this position, the unnecessary ink is discharged from all the ink-jet nozzles, for example, by means of a cleaning unit of the vacuum type. The cleaning unit includes a suction pump for creating negative pressure, and a suction tool which is brought into contact with the ends of the ink-jet nozzles to draw out the unnecessary ink by the negative pressure applied from the suction pump.
Recently, an improved ink-jet printer has been developed, which can perform multicolor-printing at a higher speed than the aforementioned printer. This type of ink-jet printer includes a rotary drum for rotating at a constant circumferential speed and an ink-jet print head for jetting color inks onto a paper sheet held on the peripheral surface of the rotary drum. Printing is performed in a condition where the paper sheet has been supplied to the rotary drum from the front side of the drum, and rolled on the drum. After the printing, the paper sheet is separated from the rotary drum, and discharged the rear side of the drum.
The print head includes nozzle units for the colors of, for example, yellow, cyan, magenta and black, which are arranged along the peripheral surface of the rotary drum. Each nozzle unit has a plurality of ink-jet nozzles lined across the paper sheet in a main scanning direction parallel to the axis of the rotary drum, and performs printing in the overall area of the paper sheet by jetting ink from the ink-jet nozzles while the paper sheet is moved in a sub-scanning direction perpendicular to the main direction in accordance with the rotation of the drum.
However, since the print head of the ink-jet printer is relatively large in size, a large space is required in the housing of the printer to move the print head for the maintenance. This is an obstacle to making a compact ink-jet printer.
Further, when the print head is moved outside the paper holding region of the rotary drum in the main scanning direction, the long distance of the movement not only hinders an increase in maintenance speed but also reduces the accuracy of positioning the print head with respect to a paper sheet, resulting in difficulty in obtaining a high-quality print.